In an IC technology and an LSI technology, for the purpose of improving operating speed and scale of integration, part of electrical wiring on an electrical wiring board is replaced with optical wiring such as an optical fiber or an optical waveguide, and an optical signal is used instead of an electrical signal.
For example, Patent Literature 1 discloses that an optical waveguide film is provided above an IC chip having an optical element provided thereon and optical communication is carried out between the IC chip and the optical waveguide film. However, when optical communication is carried out between a substrate including optical communication means such as an optical element and optical communication means such as an optical waveguide as in Patent Literature 1, there are problems in that, unless these optical communication means are positioned with high accuracy in mounting, the optical communication cannot be carried out, and in that, without light concentration, optical loss increases to reduce signal strength.
In order to solve these problems, a microlens is provided on a surface of the substrate. As a method of producing such a microlens with ease, for example, as described in Patent Literature 2, a method using so-called thermal sagging is known. Specifically, a photosensitive resin resist is formed on a front surface of a transparent substrate, and a light shielding film having an opening portion formed therein is formed on a rear surface of the substrate. Then, light is radiated from the light shielding film side to expose a portion of the photosensitive resin resist which exists at a location opposed to the opening portion in the light shielding film. After that, development is carried out to forma columnar resist structure. After that, by heating the resist structure to cause thermal sagging of a surface of the resist structure, the microlens is produced.
Further, as another method of producing a microlens with ease, a method of forming the microlens by dropping a liquid droplet of a resin on a substrate is also known.